1. Field of Invention
This invention relates to the field of electronic circuits, specifically to a power-switching circuit allowing a momentary-contact switch to control the power to an electronic device.
2. Background of the Invention
Electronic devices are traditionally turned on and off by mechanical switches that latch permanently in the on or the off state.
The miniaturization of electronics has driven designs away from mechanical switches. Portable devices such as cell phones and calculators have recently become cheaper, smaller, and more complex. In such devices, the mechanical components of a switch are prohibitively large and expensive, and an electrical switching circuit must be used instead. This is particularly true for devices with current requirements in the 1-50 ampere range, where mechanical switches might be more than ten times larger and more than ten times more expensive than their electrical counterparts. Mechanical switches form part of the user interface, so they must have visual and tactile appeal, which puts even more constraints on their design. An electrical switch can be internal and invisible to the customer, allowing more flexibility in its design.
When an electrical switch is used to control power to a device, a small mechanical switch can be used to control the electrical switch. Since the mechanical switch does not conduct the main device power, it can be small and inexpensive.
Another advantage of using an electrical switch is that the device can turn off its own power. Since the state of the switch is stored electrically, it is possible for the main circuit of the device to have an output that shuts off power. This provides an “auto-off” or “sleep” function for the device.
U.S. Pat. No. 4,538,074 (Fraden, 1985) describes a switching circuit requiring a microprocessor, two transistors, and a bilateral switch. The switching circuits in U.S. Pat. No. 3,831,081 (Weiss, 1974), U.S. Pat. No. 5,692,203 (Grodevant, 1997), and U.S. Pat. No. 6,548,996, (Yi, 2003) each employ multiple logic gates or a flip-flop. Such components add significant cost to devices using these designs, and they consume power even in the “off” state. More importantly, these components put limitations on the voltage of the power supply. A typical flip-flop requires a voltage supply of 4.75 to 5.25 volts, while battery-powered devices are built to use voltage supplies anywhere from 1 to 20 volts, and a single battery pack might, for example, vary between 4.0 and 6.5 volts during its useful life.
U.S. Pat. No. 3,255,365 (Henle, 1966) describes a simple bistable circuit using a PNP and an NPN transistor that requires separate switches for its on and off switching functions. The bistable circuit used as part of the present invention is an improved version of the bistable circuit described in that patent.